A number of peptides have been discovered which are common to the gastrointestinal tract and the brain. In the gut most of these are known or at least postulated to be important in the endocrine or paracrine regulation of digestion. Little is known about their role in the brain. Because many of these peptides are found in brain sites known to participate in the neural control of the gastrointestinal tract, we postulated that some of these peptides common to gut and brain may have related functions at their two sites of origin. We have found that injections of gastrin-like peptides into the lateral hypothalamus or medulla of the brain in doses that are too small to be effective systemically potently stimulate gastric acid secretion in the rat. The purpose of our experiments is to characterize the actions of gut and brain peptides on the neural control of gastrointestinal function, to identify their mechanisms of action and to investigate their physiological significance in the maintenance of energy balance. We will investigate and compare the effects and mechanisms of action of intracranial, intravenous and intra-arterial injections of various peptides, analogs and fragments on the gastric acid secretion, gastrointestinal motility and ingestive behaviors of conscious rats, cats and dogs.